Byproducts and conditions associated with excavation activities are known to lead to the generation of various forms of environmentally harmful pollution. As used herein, excavation activities encompass not only conventional mining operations to locate and recover natural resources below the surface of the earth, but also any other operations that disrupt large areas of the natural surface and/or contour of land. For example, highway construction and other large commercial developments often produce the same byproducts and conditions as conventional mining operations and constitute excavation activities within the scope of the present inventions.
FIG. 1 provides an exemplary drawing of an excavation activity 10 to illustrate various byproducts and conditions that may lead to the generation of various forms of environmentally harmful water pollution. The byproducts may include, for example, various forms of mineral processing waste, such as overburden 12, slag, gangue, and buried pyrite-bearing rock 14. The conditions may include, for example, the excavation site itself, draining adits, tunnels 16, abandoned pits 18, and mines. As shown in FIG. 1, rain water 20, runoff 22, and other sources of water may pass over and through various portions of the excavation activity 10 and interact and react with the byproducts and conditions to produce undesirable water pollution 24. The undesirable water pollution 24 generally gravity drains through the excavation activity 10 until it reaches an impermeable barrier, such as a natural or man-made liner 26, which eventually guides the water pollution to an outfall, such as a stream or underground well or ground water infiltration/recharge zone.
The water pollution 24 produced by excavation activities may be generically referred to as acid rock drainage (ARD) or acid mine drainage (AMD), and will hereinafter be collectively referred to as ARD. The combination of water, bacteria, and sulfide minerals exposed to air by excavation activities produces sulfuric acid, sulfates, iron and other metals in the ARD. For example, the following four generally-accepted chemical reactions describe the oxidation of sulfide minerals (represented by FeS2 as a proxy for all reactive sulfide minerals) that produces ARD:FeS2+7/2O2+H2O→Fe2++2SO42−+2H+  1.Fe2++¼O2+H+→Fe3++½H2O  2.Fe3++3H2O→Fe(OH)3+3H+  3.FeS2+14Fe3++8H2O→15Fe2++2SO42−+16H+  4.
As shown by the preceding equations, the elementary chemical ingredients required for the formation of ARD are air, water, and sulfide materials. As described below, bacteria can facilitate the formation of ARD. Once each elementary ingredient is present, the production of ARD may be predicted by a number of standard tests, including acid-base accounting tests, humidity cell tests, and column leach tests. For example, in a pH environment of less than approximately 4.5, naturally-occurring bacteria, such as acidithiobacillus ferro-oxidans and related microbes, may act as a catalyst and accelerate reactions 1, 2, and 4 above, lowering the pH even further. Hydrogen ions (H+) and ferric iron ions (Fe+3) may also accelerate the oxidation of other metal sulfides that may be present, releasing additional metals such as copper, lead, zinc, cadmium, mercury, and manganese into the ARD.
An effective method for reducing and/or preventing ARD is to remove and/or isolate one or more of the elementary ingredients—air, water, sulfide materials, and/or bacteria—required for ARD production. For example, a generally accepted system and method for treating byproducts and conditions associated with an excavation activity is to disperse one or more active ingredients or reagents over the excavation site to react with one or more of the elementary ingredients. As shown in FIG. 1, however, the byproducts and conditions associated with excavation activities are often buried, widely dispersed, and otherwise inaccessible to direct application of the active ingredients, requiring a combination of closely-spaced boreholes, gravity, and voluminous amounts of water to transport the active ingredients to the affected areas to be treated. Although effective at reducing or preventing ARD for the areas actually reached, the water-dispersed active ingredients often fail to reach all of the byproducts and conditions requiring treatment before passing through the excavation site. Therefore, an improved system and method for reliably dispersing active ingredients to treat excavation activities would be useful.